The named inventor in this application, Indresh Mathur, has been issued U.S. Pat. No. 9,005,380, which discloses and claims rocket propellants having a density of about 0.76 grams/cm3 to about 0.78 grams/cm3. The fuels disclosed in U.S. Pat. No. 9,005,380 had a high hydrogen content and consequently had a high heat of combustion on a mass basis (i.e., high energy content per unit of mass). These high mass energy density fuels have the advantage of reducing the mass of fuel needed to achieve a desired specific impulse (i.e., thrust per unit mass of fuel per unit of time). However, the advantage of higher mass energy density is not fully realized in rocket propellant applications because of the somewhat lower mass density or specific gravity of these fuels as compared with conventional rocket fuels such as RP-1 which has a mass density of 0.7990 to 0.8150 grams/cm3.
The mass of a lower mass density fuel, as described in U.S. Pat. No. 9,005,380, that can be contained in the fuel tank of a predetermined launch vehicle is less than the mass of conventional fuel (e.g., RP-1) that can be held in the same tank. This means that a substantial amount of the additional total energy available due to the higher amount of energy per unit mass of fuel is lost due to a reduction of total fuel mass that can be held in a given tank of a predetermined launch vehicle.
From the available technical literature, it would appear that for hydrocarbon fuels having a low aromatic content (needed to prevent fouling and fuel degradation in rocket engines employing regenerative cooling), there is a trade-off between specific energy of the fuel (the amount of energy released per unit mass of fuel during combustion) and the mass density of the fuel (the mass of fuel per unit of volume of the fuel). In other words, attempts at increasing the specific energy tend to reduce mass density, and attempts to increase mass density tend to reduce the specific energy of the fuel.
U.S. Pat. No. 9,005,380 discloses a hydrocarbon rocket fuel having a low concentration of aromatic compounds (less than 5% by volume) that is beneficial for reusable launch vehicles (and for certain reusable air-breathing hypersonic vehicles) in which the fuel is used as a coolant to transfer heat from rocket engine components, such as the combustion chamber and nozzle of a rocket engine. In such case, the fuel is passed through small diameter tubes or channels around the combustion chamber or nozzle of the engine. Such process is termed “regenerative cooling.” It is important that fuels used in a regenerative cooling process exhibit extraordinarily good thermal stability, i.e., that they are highly resistant to thermal degradation. Thermal degradation of a hydrocarbon fuel during regeneration cooling is undesirable because it increases the pressure drop through the tubes or channels of the combustion chamber or in the engine nozzle or other components where carbon accumulates on the tube or channel walls. Accumulation of thermal degradation products (fouling) on the regenerative cooling tubes or channels also adversely affects the overall heat transfer coefficient. Higher pressure drop and lower heat transfer rates reduce the rate of heat transfer, resulting in increased temperatures, which in turn results in an increased rate of thermal degradation of the fuel. Thus, even fuels that are generally considered to exhibit good thermal stability can cause fouling of the regenerative cooling tubes that occurs at an ever increasing rate during operation of a rocket engine, potentially leading to catastrophic failure.